It is known that SCR systems in Diesel engines are able to achieve a reduction of NOx emissions by injecting in the exhaust pipe a Diesel Emission Fluid (DEF). An example of such fluid commercially available and used in automotive application is a solution made up from 32.5% Urea mixed with de-ionized water that is maintained at a suitable pressure setpoint by a pressure regulation circuit. The DEF is injected in the exhaust gas by a dedicated injector mounted on the exhaust pipe, in such a way that it can be mixed into the exhaust gas taking advantage of the exhaust gas flow. The SCR catalyst is mounted downstream of the injector with respect to the exhaust gas flow, so that the DEF added to a stream of gas is absorbed inside the catalyst, where due to the temperature of the system the nitrogen oxides are converted according to the following chemical equation (stoichiometric reaction):4NO+2(NH2)2CO+O2→4N2+4H2O+2CO2.
A typical schematic illustration of the exhaust architecture for a known SCR system is shown in FIG. 1. The scheme includes a DEF tank provided with a DEF pressure regulator, used to maintain DEF pressure at a certain setpoint to maximize injection spray efficiency. The SCR system includes also a DEF injector to inject DEF into the exhaust pipe of the engine in order to reach the SCR catalyst. Because of the working principle of the SCR system, it is not required that the injector ensures a continuous DEF flow, but the goal of the system is to maintain a certain level of DEF absorbed in the catalyst.
SCR systems have been used in automotive applications for heavy truck applications, but these systems are now requested also for passenger car applications to fulfill Euro 5 and Euro 6 NOx emission legislations. For this reason, since DEF injectors used in SCR systems have been developed in the past for heavy truck applications, they have static flow and dynamic flow rates higher than what is now requested for passenger car applications. This means that in these latter applications, a typical DEF injector works only in the small quantity injection area of its characteristic, an area that is typically non linear, that has bad precision and robustness and that is also severely subject to aging drift.
By contrast, to develop a DEF injector with lower static flow and dynamic flow rates is very challenging also from the hydraulic perspective, due to the hole diameter and to the spray geometry, this latter factor being dependent upon the number of holes.
In addition, recent legislation requires that a deviation of more than 50% between the average reagent consumption and the average demanded reagent consumption by the engine system over a period of 30 minutes of vehicle operation shall result in the activation of the driver warning system. Tighter standards may be imposed in the future.
In SCR systems, a known NOx reduction strategy, using as input a NOx sensor, request a certain injection mass for the injection strategy, with a certain refresh period. The injection strategy transforms the input, namely the mass request, in an output for the injector that typically is the opening time for the injector. The assumption is that the injector can be driven with a minimum injection period between one injection and the next one, and that two injections can be distant N times this minimum injection period.
In a hypothetical linear behavior of the injector, the requested DEF injection quantity is transformed in an opening time for the injector (directly from the characteristic of the DEF injector) and eventually corrected via a calibration map. But, in some engine operating points, depending also on the calibration of the NOx reduction strategy and on its model behavior, the requested DEF injection mass can be so little that the opening time of the injector is below a minimum mechanical threshold and the injected quantity can be completely different from the request. Injected quantity may even be zero in the case that the injector remains hydraulically closed. In general, it may be said that in these conditions for little quantities of requested DEF the injector behavior is not linear. Also, the DEF injector behavior gets worse with injector aging. This problem causes errors in the estimation of the DEF injected mass, and has a negative influence on emissions and also jeopardizes the fulfillment of emission legislation.
In view of the foregoing, at least a first object is to provide a method for controlling DEF injected quantity in a NOx reduction system employing a SCR catalyst that allows for the injection of correct quantities of fluid in all situations of use of the vehicle. At least another object is to provide a method for controlling a DEF injected quantity that allows an optimal control of such fluid in passenger cars. At least another object is to provide a method for controlling the DEF injected quantity without using complex devices and by taking advantage of the computational capabilities of the Electronic Control Unit (ECU) of the vehicle, and at least another object of the present invention is to meet these goals by means of a simple, rational and inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.